The invention relates to a reactor with improved heat transfer and a process for carrying out an overall endothermic gas phase chemical reaction(s) in this reactor. The invention relates in particular to a radiating wall catalytic reactor for providing heat from the inside wall of a reaction chamber by radiation to support an endothermic reaction taking place in the reaction chamber, comprising a reaction chamber with an entrance port for the introduction of a gaseous reactant(s) in a continuous manner into the reaction chamber and an exit port to enable the gaseous product to leave the reaction chamber in a continuous manner. The invention relates also to a method for carrying out a chemical reaction in this reactor involving an improvement in heat transfer by radiation.
Chemical reactions carried out in such a reactor would usually involve the transfer of a large amount of heat through the wall of the reaction chamber (for example in Heaters for Chemical Reactors by D. Lihou, 1975, heat fluxes in the range of 15 to 100 kW/m2 of tube surface area which contains the catalyst are reported). If the reaction is highly endothermic, then sufficient heating must be provided to sustain the desired reactions and this can be provided in a variety of ways. When the temperature of the reaction needs to be maintained high (e.g. above 700° C.), then the reaction chamber may be in the form of a tube (containing the catalytic material) which is mounted inside a fuel (gas or liquid fuel) fired furnace, which provides the necessary external source of heat from a hot zone (e.g. 1200 to 1500° C.) in the furnace maintained by burning a fuel. Heat from this flame and neighboring gaseous hot zone is transferred by a combination of radiation and convection to the outside surface of the tube (reaction chamber). There would normally be a plurality of tubes (reaction chambers) inside such a furnace. Within an individual tube which acts as a reactor, heat is usually transferred from the inside wall of the tube, to the flowing fluid and catalytic bed by a combination of: convection, conduction and radiation. In the hitherto known reactors, a variation of the temperature occurs over the cross-section of the tube (reaction chamber) which could be significant and then it has several drawbacks. Namely, the outside surface of the reaction chamber might need to be heated to very high temperatures to assure a sufficiently high temperature in the interior of the reaction chamber for desired endothermic chemical reactions. However, the life time of the reactor material strongly depends on the temperature and decreases sharply once a too high temperature is reached. On the other hand, at elevated temperatures near the wall, undesired chemical reactions might take place. For example, these could lead to the formation of carbon deposits, which would increase the resistance to heat transfer at a local position, which in turn could then lead to hot spots near the inner wall of the reaction chamber. Or, large temperature gradients from the wall to the centre of the tube (the reactor) can result in lower temperature regions in the centre of the tube (the reactor), which could lead to incomplete reactions near the centre, which in turn would require a longer length of reactor to achieve the necessary conversion.
Various attempts have been undertaken to overcome this situation and to alleviate the effects associated therewith.
U.S. Pat. No. 4,042,334 discloses a high temperature chemical reactor which comprises a tube which defines a reactor chamber, means for introducing an inert fluid into the reactor tube to provide a protective blanket for the inside surface of the tube; means for introducing reactants into the chamber, the reactants being confined centrally within the chamber by the protective blanket; and means for generating high intensity radiant energy which is directed in the chamber to coincide with at least a portion of the path of the reactants.
U.S. Pat. No. 5,322,116 describes a high temperature fluid-to-fluid heat exchanger for transferring heat from a higher temperature fluid flow region to a lower temperature fluid flow region, comprising inter alia wall means separating the high and lower temperature fluid flow regions and a porous ceramic foam material occupying a substantial portion of the lower temperature fluid flow region. The ceramic foam material is positioned proximate said wall means to absorb a substantial amount of radiated heat therefrom, wherein said ceramic foam does not contact said wall means, such that a narrow gap is formed between the wall means and the ceramic foam material having a porosity sufficient to permit a predetermined flow rate of fluid along the edge thereof.
Nijemeisland et al (2004), describe how computer fluid dynamic simulation could be used to model heat transfer performance in the near-wall regions of a steam methane reforming reactor, which is packed with catalyst pellets. This paper is interesting as one of the co-authors is from Johnson Matthey Catalysts (UK), a major company that manufactures and sells commercial catalysts for this type of application. In their paper (p. 5186), they state that “radiation was neglected in the simulations presented here, as it was shown in earlier work to be insignificant compared to convective heat transfer processes.” This is an important statement, as it provides evidence that such methods of heat transfer are considered by such experts to be predominantly convective in nature.
M. Nijemeisland, A G Dixon, E. H. Stitt, (2004) Catalyst design by CFD for heat transfer and reaction in steam reforming, Chemical Engineering Science, 59 (2004) 5185-5191.
WO 2009/109379 A1 discloses a reactor, which is an endothermic catalytic reactor comprising one or more reactor tubes (or “ducts”) which are formed by an inner and an outer tube/duct, which creates an annular region in the duct. The inner tube/duct comprises a catalyst, e.g. particles forming a catalyst fixed bed or catalyst coated or impregnated on structural elements arranged in the inner duct, whereby the structural element is for example a monolith. Disclosed are in particular monoliths coated with a noble-metal catalyst and a Ni-based catalyst. In between the catalysts, a flow detector is arranged, which forces the fluid passing through the outer duct (annular region) to flow inside the inner duct, which contains the catalyst. At the same time, the flow detector also forces the fluid passing through the inner duct to flow inside the outer duct. The reactor tube comprises for example several tube segments, e.g. fourteen tube segments, whereby each tube segment comprises a catalyst, an annulus and a flow deflector. One substream of the fluid thus passes the outer duct, i.e. the annular region, is forced by the flow detector into the inner duct with the catalyst and, after passing the catalyst, it is forced by the next flow detector into the outer duct again, whereas another substream of the fluid passes the same the inner duct, is forced by the flow detector into the outer duct and then by the next flow detector into the inner duct again. Thus, each substream of the fluid passes alternately the catalyst and the outer duct, thereby passing the catalyst of every second tube segment of the reactor tube. The substream running through the annular region is heated from an external source, while the substream running through the inner tube is cooled via endothermic reforming taken place on e.g. the catalytic structural elements. Thus, the substream, which passed the inner duct with the catalyst has been cooled therein and is then heated when passing the outer duct. The catalytic reactor may be used for steam reforming.
US 2011/0 194 991 A1 discloses a tubular reactor comprising an internal catalytic insert with a plurality of cup-shaped structures, whereby a supported catalyst is disposed or located within the central space of each of the plurality of the cup-shaped structures. In between two cup-shaped structures there may be a distance. The cup-shaped structures have orifices for forming fluid jets for impinging the fluid on the tube wall. This jet impingement is used to improve heat transfer between the fluid in the tube and the tube wall. Such a tubular reactor may be used for instance for endothermic reactions such as steam methane reforming.
WO 2008/040999 A2 discloses a reactor for carrying out a heterogeneously catalyzed reaction of at least one gaseous reactant and at least one liquid phase reactant. The reactor comprises at least first and second reaction zones, which are arranged in series and each comprising catalytic material. The catalytic material may be in particulate form or in the form of a unitary body defining pathways for the reactant gas/liquid mixture, whereby in a specially preferred embodiment the reaction zones each comprise a structured catalyst bed in the form of a monolith with parallel channels, e.g. a Pt catalyst distributed on a carbon support in the form of a monolith. Between the serially arranged reaction zones, heat transfer zones are located which may be of any suitable structure having regard to their function of helping to maintain the temperature of the contents of the reactor within desired limits. In general, the heat transfer zones define enclosed channels in which heat transfer fluid can flow in isolation from, but in heat-exchange relationship with, the process fluid of the reactor. Suitable heat transfer devices for use in the heat transfer zones include tubes or compact heat exchanger types of plates.
U.S. Pat. No. 3,617,227 discloses an apparatus for the catalytic reforming of gaseous hydrocarbon using steam and/or carbon dioxide as the reforming oxidant, e.g. steam methane reforming. The hydrocarbon and oxidant are passed through a tube, which is substantially uniformly heated over its entire length, e.g. to 1800-2200° F., and which contains refractory preheater particles adjacent the inlet opening and catalytic particles intermediate the preheater particles and the tube outlet. The catalyst is for example a 2-inch to 3-inch Alundum lump impregnated with nickel. If the catalyst tube is heated, the catalyst lump and the preheater lump are heated indirectly by radiation and conduction. By having the catalyst lump large, the individual particles are exposed directly to radiation emitted from the tube and are thereby able to benefit directly from the source of heat.